Advanced Graphite Creep (AGC) Experiment

Will WindesIdaho National LaboratoryART Graphite R&D Technical Lead

GCR Review MeetingIdaho National Laboratory14-16 July, 2020

This work was performed at Idaho National Laboratory (INL) by Battelle Energy Alliance, LLC under contract DE-AC07-05ID14517 and Oak Ridge National Laboratory (ORNL) by UT-Battelle, LLC, under contract DE-AC05-00OR22725.

Also would like to thank Mr. W. David Swank, Mr. David Rohrbaugh, and Dr. Tim Burchell for providing data and results within their research activities for this presentation.

Acknowledgements

2

Outline

Outline• Brief description of AGC

Original AGC Experiment objectives New (2018) high dose plans

• Experiment status AGC-4 and HDG-1 status

• Analysis of AGC results Initial 600°C data (AGC-1 & -2 data) New 800°C data (AGC-3) creep data

• What do you do with this data? Where does the data fit in? What will commercial vendors still need

to do? ASME qualification (briefly)

• Conclusions

Graphite R&D Program

Defines the safe working envelope for nuclear graphite

and protection of fuel

Behavior models- Predicts irradiated

material properties and potential degradation issues

- Irradiation behavior for continued safe operation

Mechanisms and Analysis- Data analysis and interpretation- Understanding the damage mechanisms

is key to interpreting data

Irradiation- Determines irradiation

changes to material properties- Irradiation behavior for

continued safe operation

As-Manufactured Properties- (Statistically) Establishes as-received

material properties- Baseline data used to determine irradiation

material properties

Licensing & Code- Establishes an ASME

approved code (for 1st time)- Develops property values for

initial components and irradiation induced changes

Five different research areas

• Irradiated material property changes (including creep) Six major grades, all properties needed for qualification (including creep rate), irradiation behavior

• VHTR conditions (1000°C outlet, 7 dpa dose) HTR conditions (<800°C outlet, 15 dpa dose)

Turnaround dose is critical for nearly all material properties

• However, we do lose temperature dependence 5

The AGC Experiment

New HDG capsules AGC-2 samples loaded into HDG-1 HDG-1 capsule assembled 600°C & 15 dpa (total)

HDG-1 ready for irradiation in ATR Sample loading order for HDG-2

From AGC-3 & AGC-4 specimens 800°C & 15 dpa (total)

Completed initial 600°C and 800°C irradiation AGC-1 and AGC-2 (600°C, 7 dpa) AGC-3 and AGC-4 (800°C, 8 dpa)

Completed initial PIE AGC-1 and AGC-2 (600°C, 7 dpa) AGC-3 (800°C, 8 dpa) - AGC-4 in 2021/2022

Re-irradiate select samples AGC-1 / AGC-2 HDG-1 AGC-3 / AGC-4 HDG-2

Status of AGC Irradiation

0 2 4 6 8 10 12 14 16ε,

σ, ρ

, α, E

, CTE

Dose, dpa

Data from AGC Experiment

How AGC data is reported (each capsule) Characterization plan Pre-IE data package As-Run Irradiation report Data qualification report Disassembly report PIE data package Irr. creep analysis report Irr. material property analysis report

• All reports are publicly released All in OSTI database

• All reports contained within NDMAS Incorporated in NDMAS Portal

• Data linked to specific reports • Reports needed to understand data in

database AGC data on NDMAS portal after Baseline

data incorporated

W.E. Windes, T.D. Burchell, M. Davenport, “The Advanced Reactor Technologies (ART) Graphite R&D Program”, Nuclear Engineering Design, 362 (2020), NED 110586, ISSN 0029-5493

William E. Windes David T. Rohrbaugh W. David Swank, "AGC-3 Irradiation Creep Strain Data Analysis", INL/EXT-19-54725, July 2019

William E. Windes, David T. Rohrbaugh, W. David Swank,"AGC-3 Irradiated Material Properties Analysis", INL/EXT-20-58029, April 2020

Dimensional change and irradiation creep

• Critical data to determine lifetime of core components Required to determine the internal stress build-up inside

components• Creep relieves this stress!

Required to determine structural integrity• Information needed for ASME graphite code• Compared to the mechanical strength of component

• Critical for determining core performance Gaps change between core components

• Coolant by-pass increases• Core thermal conductivity changes

Restraints, keys, control rod insertion

• Determining turnaround dose is key Turnaround dose is key to understanding all pertinent material

property changes Point where microstructure catches up to atomic irradiation

damage

Let’s look at some AGC data (1/2)

8

-4

-3

-2

-1

0

0 1 2 3 4 5 6 7 8

ΔL/L

Dose, dpa

Dimensional Change and Creep (600°C)

600°C

600°C

Effects of temperature on dimensional change & creep response

• Temperature affects both dimensional change and creep rate Higher temperatures yield shorter turnaround dose

(not shown here). But higher temperatures also yield higher creep rates

• Higher rate of stress relief

• Implies significantly different approach to core design for different temperatures Higher temperatures usually means shorter

component lifetime • Lower turnaround dose

• BUT : Lower stress buildup within component• Component gaps grow faster• Core efficiency degrades faster

Lower temperatures slow everything down• Higher turnaround dose – but larger internal stress• Less stress relief from creep

9

-5

-4

-3

-2

-1

0

0 1 2 3 4 5 6 7 8ΔL

/LDose, dpa

Temperature effects on dimensional change (600°C & 800°C)

600°C 600°C

800°C 800°C

Let’s look at some AGC data (2/2)

All vendors must establish turnaround dose for their graphite grade

Turnaround dose – critical parameter

10

-10

-5

0

5

10

15

20

0 5 10 15 20 25

Dim

ensi

onal

Cha

nge,

% (Δ

V/V)

Dose, dpa

Dimensional change vs. neutron dose

PCEA (750C) PCEA (950C)

IG-110 (750C) IG-110 (950C)

• Why do we care? Point where irradiation induced material

property changes begin to reverse. Point where microstructural densification

stops. Microcracking begins.

• Think of “before” and “after” turnaround Behavior is much more predictable for all

graphite grades before turnaround• Much less predictable (more data scatter)

after turnaround Crack propagation retarded in compressive

stress fields. Crack propagation accelerated in tensile

• Turnaround dose changes significantly with temperature IG-110 (50µm) 10 dpa to 5 dpa PCEA (1800µm) 11 dpa to 6 dpa

From: M.C.R. Heijna, S. de Groot, J.A. Vreeling, "Comparison of irradiation behaviour of HTR graphite grades", Journal of Nuclear Materials 492 (2017) 148e156

11

Other material properties of interest (1/2)

Density

Electrical Resistivity

Modulus

Shear

Mechanical changes

• Densification reduces crack propagation Directly related to dimensional change Only property to gradually change as a function

of increased dose

• Electrical resistivity is a good measure of isotropy within crystal structure Anisotropic behavior should be avoided Anisotropy increases more after turnaround

• Young’s and Shear Modulus important to determining component structural integrity and creep rate Immediate increase due to irradiation damage

pinning dislocation movement Gradual rise with increasing dose indicates

microstructural effects

Thermal changes

• Thermal expansion is important to structural integrity, thermal performance, and core assemblies Thermal induced stresses across large components Gaps between components will change

• Physical damage from contact• Coolant by-pass issues

Stability of stacked components in core assemblies

• Thermal diffusivity needed for heat removal from fuel, core performance, and component structural integrity Increases thermal induced stresses from temperature increase Decay heat removal issues with lower diffusivity Only property to exhibit immediate and significant decrease Initial decrease occurs at << 0.1 dpa

• Then only small decrease with increasing dose Conductivity (K = 𝛼𝛼 � 𝜌𝜌 � 𝐶𝐶𝑝𝑝 ) is related to density which changes with dose

12

Other material properties of interest (2/2)

CTE

Thermal Diffusivity

Irradiation induced changes must be considered in design

• Significant changes occur during normal operation in: Dimensional change

• Turnaround dose is key parameter• Highly temperature dependent

Density• After Turnaround density decreases (volumetric expansion)• Formation of microcracks

Strength and modulus• Graphite gets stronger with irradiation …• Until Turnaround dose is achieved, it then decreases

Thermal conductivity• Decreases almost immediately to ~30% of unirradiated values

Coefficient of thermal expansion• Initial increase but then reduces before Turnaround• CTE is why properties are so temperature dependent

• Significant changes do not typically occur in the following properties: Oxidation rate, neutron moderation, specific heat capacity, emissivity, heat capacity 13

Irradiation Effects on Graphite Properties

Dimensional Change

Strength & Modulus

CTE

Thermal Diffusivity

Graphite R&D Program

Defines the safe working envelope for nuclear graphite

and protection of fuel

Behavior models- Predicts irradiated

material properties and potential degradation issues

- Irradiation behavior for continued safe operation

Mechanisms and Analysis- Data analysis and interpretation- Understanding the damage mechanisms

is key to interpreting data

Irradiation- Determines irradiation

changes to material properties- Irradiation behavior for

continued safe operation

As-Manufactured Properties- (Statistically) Establishes as-received

material properties- Baseline data used to determine irradiation

material properties

Licensing & Code- Establishes an ASME

approved code (for 1st time)- Develops property values for

initial components and irradiation induced changes

Five different research areas

Behavior models- Predicts irradiated material properties and potential degradation issues- Irradiation behavior for continued safe operation

Mechanisms and Analysis- Data analysis and interpretation- Understanding mechanisms is key to interpreting data

Irradiation- Determines irradiation changes to material properties- Irradiation behavior for continued safe operation

As-Manufactured Properties- (Statistically) Establishes as-received material properties- Baseline data used to determine irradiation material properties

Licensing & Code- Establishes an ASME approved code (for 1st time)- Develops property values for initial components and irradiation induced changes

Qualification of graphite

Degradation- Oxidation or molten salt effects- Abrasion, erosion, fatigue

Qualified

(Matthews)

(Geringer)

(R. Smith)

(Gallego)

(J. Bass)

• AGC irradiation program about 75% complete Only the high dose (HDG) capsules remaining Data from initial AGC capsules can be used for micro, small modular,

and prismatic designs right now

16

Conclusions

• Critical irradiation data Turnaround dose must be established Operating temperatures influence component qualification

• and core design considerations (lifetime, replacement, degradation) Designing beyond turnaround dose will be difficult and expensive (in

money and time) AGC data available through INL’s NDMAS database (soon)

• Qualifying specific graphite grades Use of ASME BPVC would be best As-fabricated material property testing is required May not need to get irradiated, oxidation, and molten salt data

• Depends upon Rx design and which graphite is chosen Must have expertise in graphite before license application

17